Police in London said early Sunday that a second suspect has been arrested in connection with Friday's London subway attack.
A 21-year-old man was arrested late Saturday night in Hounslow in west London under the Terrorism Act, authorities said. No additional information was immediately available about the second suspect.

Previously, an 18-year-old man was arrested for possible involvement in the attack that injured 29 people.

Penelope Jones and Ronald Jones as they receive their MBEs from Queen Elizabeth II at Buckingham Palace.
(Lewis Whyld/PA Wire)

Despite the arrests, Britain's terror threat level remained at "critical" -- the highest level -- meaning authorities believe another attack may be imminent.

On Saturday, police scoured the country in search of possible suspects. Their efforts led them to the suburban London home of an elderly couple honored for their work with child refugees.

The property was registered to Ronald Jones, 88, and his 71-year-old wife, Penelope, who have been honored by Queen Elizabeth after taking care of 268 foster children, the Times of London reported.

Police officers stand behind cordon tape near a property that was searched after an explosion on a London
Underground train, in Sunbury-on-Thames, Britain, September 16, 2017.
REUTERS/Peter Nicholls - RC198DFC0FE0

Neighbors told the Sun that the couple have six children of their own and had started taking foster children after seeing the plight of refugees displaced from Syria and elsewhere.

The 18-year-old, whom authorities tracked using facial recognition technology, was believed to be a foster child taken in by Jones and his wife, according to the Sun, which also quoted a neighbor saying the elderly couple's foster children were Syrian.

The massive search at the house in Sunbury-on-Thames, about five miles from London’s Heathrow Airport follows the “very significant arrest” of an 18-year-old in Dover.

Police showed up at the house, which is in Surrey, and ordered an immediate evacuation of neighboring homes.

Mojgan Jamali, who lives near the house being searched, said police gave her "one minute" to pack.

"I was in my house with my children and there was a knock at the door from the police. They told me to leave. They said, 'You have one minute to get out of the house and get away,'" she said. "I just got out. I got my three children and we left the house and the street."

Some neighbors were crying and many said they had no idea when they would be able to return to their homes. Police gave many shelter and food at a nearby sports club.

Police did not reveal details about the search, but the precautions suggested concern that there might be explosives or violent extremists on the property.

The Islamic State group has claimed one of its units planted the bomb.

Police officers stand behind cordon tape near a property that was searched after a man was arrested in
connection with an explosion on a London Underground train,
in Sunbury-on-Thames, Britain, September 16, 2017.
REUTERS/Peter Nicholls - RC1D0656CBA0

Police said the teen was arrested by Kent police in the port area of Dover on the English Channel.

The suspect was being held for questioning under the Terrorism Act. He has not been charged or identified. Police have not said if he was suspected of planting the bomb or if he played a supporting role in a possible plot.

British Home Secretary Amber Rudd said it was “much too early” to tell if the authorities knew of the suspect in the London subway bombing.

Rudd said police were making rapid progress in the investigation.

Hundreds of officers were examining surveillance footage and conducting other investigations as the nation elevated its terrorism alert system to the highest level, the BBC reported.

Authorities said the suspect carried a white bucket containing an explosive onto the rush-hour train. When it exploded, numerous train riders suffered burns, and others were injured as they rushed away from the area of the blast.

None of the injuries were believed to be life-threatening.

The Islamic State group claimed responsibility for the attack, which it said was carried out by an affiliated unit.

Meanwhile, President Trump took heat from British authorities for a tweet he posted in the wake of the attack that seemed to criticize Britain’s security forces.

“Another attack in London by a loser terrorist. These are sick and demented people who were in the sights of Scotland Yard. Must be proactive!” the president tweeted.

Reuters reported that British Prime Minister Teresa May responded: “I never think it’s helpful for anybody to speculate on what is an ongoing investigation.”

Officials have hinted that more than one person may have been involved.

May said raising the threat level to its highest point was a "proportionate and sensible step." Police called on the public to be vigilant.

The bomb went off around 8:20 a.m., local time, as the train, carrying commuters from the suburbs -- including many school children, was at Parsons Green station in the southwest of the city.

The station was reopened Saturday, officials said, restoring some normalcy to London's transport network after a day of disruption. There was no sign of panic among Londoners and the weekend life of the city continued undeterred by the raised threat level.

Analysts said the injuries would have been far worse had the entire device exploded.

Britain has endured four other attacks this year, which have killed a total of 36 people.